Conventionally, various types of cooling structures with improved cooling performance for a cylinder head of a multi-cylinder internal combustion engine have been proposed. For example, Patent Document 1 discloses a cooling structure in which a coolant passage in a cylinder head is divided into an intake-side passage and an exhaust-side passage, and the exhaust-side passage is divided into exhaust-side lateral passages and an exhaust-side longitudinal passage. The intake-side passage is arranged below intake ports and extends along the arranging direction of cylinders. Each of the exhaust-side lateral passages is provided between a corresponding adjacent pair of the cylinders and extends along a direction substantially perpendicular to the arranging direction of the cylinders. A coolant inlet of each exhaust-side lateral passage is arranged between the corresponding adjacent cylinders and in the vicinity of the intake-side passage. The exhaust-side longitudinal passage is provided in the vicinities of exhaust ports and extends in the arranging direction of the cylinders. The downstream end of each exhaust-side lateral passage is connected to the exhaust-side longitudinal passage.
In the cooling structure, a sufficient flow of low-temperature coolant is sent from a radiator to the intake-side passage. The coolant thus cools the intake air flowing through the intake ports, suppressing knocking. Further, since an upstream side of the exhaust-side passage is formed by the exhaust-side lateral passages, a sufficient amount of coolant is provided to heated portions corresponding to the sections between the cylinders. The heated portions are thus efficiently cooled.
Each of the exhaust-side lateral passages and the inlet of the exhaust-side lateral passage of this cooling structure of the cylinder head are provided between the corresponding adjacent pair of the cylinders. This arrangement allows the coolant flowing in the exhaust-side lateral passages to cool the portions corresponding to the sections between the cylinders and the vicinities of these portions, which are, for example, the portions in the proximities of combustion chambers. However, the cooling structure does not appropriately send the coolant to the vicinity of the top of each of the combustion chambers, which are formed in the respective cylinders. This makes it difficult for the coolant to cool, particularly, most heated portions such as a section between each adjacent pair of the exhaust ports, a section between each adjacent pair of the exhaust valves, and the proximity of the ignition plug of each of the cylinders.
Patent Document 1: Japanese Utility Model Registration No. 2526038